The sources of renewable energy, mainly wind power plants (WIN) and photovoltaic (PH) power plants, have ceased to be a marginal resource in the generation of electricity.
The generators of PH plants use power converters to connect to the electric grid. These power converters, also known as inverters, inject current to the voltage of the grid to supply power to the same.
In normal operating conditions, the PH inverters inject single-phase or tri-phase sinusoidal currents to the grid, depending on the power range of the generator. Normally, the currents injected to the grid tend to be sinusoidal and are in phase with the voltages of the coupling point to maximize the amount of active power being generated. There are no PH inverters in the market, at least in on a generalized basis, that inject currents to the quadrature with the grid voltages, which allows controlling the reactive power injected to the grid with the purpose of regulating the voltage level at the coupling point. When the grid voltage is affected by a perturbation, such as imbalances, transients, or harmonics, which is usual in electric grids, conventional PH inverters experience problems to remain appropriately synchronized with the grid voltage, which leads to uncontrolled power flows that cause the PH inverter to worsen the situation of the failure of the grid. With more serious grid perturbations, such as voltage dips, short-circuits, or power oscillations, conventional PH inverters cannot offer an appropriate support to the electric grid to help maintain the generation system active. In fact, these serious transient perturbations usually cause the disconnection of the grid in the majority of the commercial PH inverters due to the triggering of some of its overcurrent or overvoltage protections. The problem caused by this type of errant behavior in conventional PH generation systems is more marked in weak grids or in grids with a high percentage of PH plants installed, which renders them unstable.
A scenario as the one described above does not offer reliability to power system operators—internationally known as TSO (Transmission System Operator), which forces them to plan and provide active and reactive power reserves to the grid by means of conventional synchronous generation or other mechanisms, to decrease the risk of the collapse of the electric system.
For obvious reasons, these power reserves are not free, which implies an additional economic burden associated with the increase in the penetration of PH generation plants.
In order to face this situation of instability, TSOs worldwide are increasingly becoming more demanding with respect to the features offered by renewable energy-based distributed generation plants. These requirements are materialized in the so-called grid codes. These types of codes are usually applied to generation technologies with a significant presence in the electric system. A clear example is the strict grid connection codes applied to wind generation systems, the PH systems being the next candidates to be regulated by these types of codes. By means of the strict requirements stipulated in the grid codes, TSOs intend for PH plants to increase their functionality and reliability with the purpose of avoiding having to pay third parties for auxiliary services that allow guaranteeing the stability of the electric system. In this case, the manufacturers and developers of PH systems are in charge of modernizing their technology to contribute the required systems, which will ultimately allow increasing the penetration of the PH energy systems in the electric grids in the following decade.
Part of the research lines found in the state of the art to improve the connection of static power converters to the grid, by authors such as Qing-Chang Zhong and Lenart Harnefors, are based on observing the operating principle of a synchronous generator and replicating it by means of the use of a static power converter.
A series of selected articles from these authors is listed below:                Zhong, Q; Weiss, G; “Synchronverters: Inverters that Mimic Synchronous Generators,” Industrial Electronics, IEEE Transactions on, vol. PP, no. 99, pp. 1, 2010.        Weiss, G.; Qing-Chang Zhong; “Static synchronous generators”; Patent, PCT/GB20091051460; WO 20101055322 A2. International Filing Date: 12 Nov. 2008.        Harnefors, L.; “Control of a voltage source converter using synchronous machine emulation”; Patent, PCT/EP20081061147; WO 20101022766 A1. International Filing Date: 26 Aug. 2008.        Lidong Zhang; Harnefors, L.; Nee, H.-P.; “Power-Synchronization Control of Grid-Connected Voltage-Source Converters,” Power Systems, IEEE Transactions on, vol. 25, no. 2, pp. 809-820, May 2010.        
Other authors, such as Josep M. Guerrero or Karel de Brabandere implement active and reactive power controllers for applications wherein the distributed power converters constitute small low-voltage grids, such as the case of micro-grids supplied by multiple uninterrupted supply systems. These controllers are characterized in that they establish the voltage reference of the power converters. Some representative articles by these authors are the following:                J. M. Guerrero, J. C. Vasquez, J. Matas, L. Garcia de Vicuña, and M. Castilla, “Hierarchical Control of Droop—Controlled AC and DC Microgrids—A General Approach Towards Standardization,” IEEE Trans Ind Electronics, 2010.        De Brabandere, K.; “Voltage and frequency droop control in low voltage grids by distributed generators with inverter front-end”; Doctoral Thesis, Leuven, België, October 2006, ISBN 90-5682-745-6.        
In terms of the patents applied for, international application WO2010055322(A2) by Weiss George [IL]; Zhong Qing-Chang [GB], can be considered as the closest to the invention object of this patent, given that it relates to a controller that faithfully emulates the behavior of a conventional synchronous generator.
The majority of the research papers mentioned above detect a series of problems derived from the attempt to faithfully replicate the operation of a conventional synchronous generator, without intending to solve its inherent inconveniences, among which we have the instability inherent to the synchronous generator, the difficulty in maintaining the synchronism during failures, or the appearance of resonance with other elements of the grid.
In this sense, the invention object of this patent supposes a solution to the problems represented by commercial inverters and conventional renewable power plants when they operate under generic conditions in the electric grid by offering the following:                Optimized response in the case of a drop/rise in the frequency of the grid, by controlling the delivered/received active power such that it limits said variation.        Optimized response in the case of a drop/rise in the effective voltage, by controlling the inductive/capacitive reactive power such that it limits said variation.        Optimized response in the case of the perturbation generated by the increase/reduction of nearby charges, supporting their supply by means of a point increase/reduction of the current injected to the grid.        Optimized response in the case of a distortion in the form of a voltage wave, affected by harmonics, transients, or imbalances, offering voltage/current conditioning functions that minimize and damp said perturbations.        Optimized response in the case of oscillations in the frequency and active power of the system, offering damping functions that minimize said oscillations in the electric system.        